Trellis-coded modulation, also referred to as trellis code, is a power-efficient and spectral-efficient combined coding and modulation technique. The technique is most useful in a communication system that uses a signal constellation other than 2-PAM (Pulse Amplitude Modulation), 2-PSK (Phase Shift Keying), or 4-PSK to deliver a higher data rate for a given signal bandwidth.
Trellis codes may be divided into two categories. The first category uses one or two-dimensional signal constellations whose signal points have one or two coordinates. And the second category uses multi-dimensional or more-than-two-dimensional signal constellations whose signal points have more than two coordinates. Note that, although the signal point in the second category has more than two coordinates, those coordinates are typically transmitted one or two each time, depending on whether the modulator uses amplitude or quadrature-amplitude modulation (QAM).
A major advantage of the multi-dimensional trellis code over one- or two-dimensional trellis code is that the multi-dimensional trellis code generates fewer redundant bits. As a result, for a given constellation size of the modulator, the multi-dimensional trellis code can deliver a higher data rate. Or equivalently, for a given data rate, the multi-dimensional trellis code requires a smaller constellation in the modulator. Generally speaking, the smaller the constellation is, the more robust the communication system is. Multi-dimensional trellis codes are therefore the preferred trellis codes when there are no other system issues involved (such as inter-symbol interference to be further discussed below).
Since mid-1980s, trellis codes have been successfully applied to many popular communication systems. To mention some of them, a two-dimensional 8-state trellis code was first adopted in ITU (International Telecommunications Union) standards on telephone-line modems at rates up to 14.4 kbps in the 1980s. Three four-dimensional trellis codes were then adopted in ITU standards on all of the more advanced telephone-line modems at rates up to 56 kbps in the 1990s. Meanwhile, a four-dimensional 16-state trellis code was adopted in an ITU standard on the asymmetrical digital subscriber line at rates up to 8 Mbps, and a four-dimensional 8-state trellis code was adopted in an IEEE standard on the gigabit ethernet. The number of states of a trellis code refers to the number of states in its encoder, which is a finite-state machine.
An important issue that is involved in all of the communication systems mentioned above is how to deal with the inter-symbol interference (ISI) introduced by the multi-path communication channel. In the telephone-line modems at rates up to 14.4 kbps, the ISI is mild and can be handled by using a feedforward equalizer in the receiver, which does not pose any problem to the communication system with a trellis code. In the more advanced telephone-line modems at rates up to 56 kbps, the ISI is quite severe because of the higher signaling rate. Fortunately, telephone-line modem is a point-to-point communication system and the communication channel is quite stationary. In that case, the transmitter can use a precoder to pre-compensate the ISI to be introduced by the channel, which again does not pose any problem to the communication system with a trellis code. In the asymmetrical digital subscriber lines, the ISI issue is avoided by using an orthogonal frequency division multiplexing (OFDM) modulation technique, together with a dynamic bit allocation method that allocates different number of bits to different frequencies.
In the gigabit ethernet, the ISI is also severe. Because the ethernet is supposed to be a point-to-multi-point communication system, the precoding technique cannot be used. As a result, a decision feedback equalizer (DFE) has to be used in the receiver. Due to the error propagation effect of the DFE, the DFE has to be used in a joint manner with the four-dimensional trellis decoder. This seems to pose a problem. However, the gigabit ethernet uses four twisted pairs for transmission. The four coordinates of each 4-dimensional signal point generated by the four-dimensional trellis encoder are transmitted in four different twisted pairs at the same time. In that case, a conventional joint DFE and trellis decoder, such as those described in U.S. Pat. No. 6,151,370, “Path-Oriented Decoder for Signal-Dependent Noise,” issued in November 2000, can be used. The operation of the joint DFE and four-dimensional trellis decoder for the gigabit ethernet is based on a conventional single-stage trellis diagram. In that diagram, each transition from a current state to a next state is associated with a four-dimensional subset of the four-dimensional signal constellation. With that single-stage trellis diagram, reliable tentative decisions on the past four-dimensional signal points can be made and used in the DFE to remove their interferences on the present received four-dimensional signal point before any further decoding processing on the present received four-dimensional signal point takes place. There is no need to worry about the interferences among the coordinates of each 4-dimensional signal point in the DFE since they are transmitted at the same time. The use of four twisted pairs in the gigabit ethernet thus avoids a potential conflict between the DFE and mult-dimensional trellis decoder in the receiver.
It would be desirable to keep using and enjoying the benefits of mult-dimensional trellis codes in other communication systems. However, in case where the communication channel has severe ISI, the transmitter cannot use a preceding technique to pre-compensate the ISI, and there are no multiple channels available to transmit all the coordinates of a mult-dimensional signal point at the same time, is it still possible to do joint decision feedback equalization and mult-dimensional trellis decoding in an effective manner? This is the question to be answered by the present invention.